U.S. Pat. No. 6,441,558 discloses a LED luminary system for providing power to LED light sources to generate a desired light color. The system comprises a controller for controlling a supply or power to the LEDs. The controller comprises two parts. The first part measures a temperature of the arrangements of LEDs, it determines a junction temperature of the semiconductors for each distinguished color, and it determines a feed forward junction temperature compensation to provide an intermediate control signal which is supplied to a lumen output module, emitting a wanted output power or lumen output for each color. A second part of said controller comprises a feedback loop, which receives the output of the lumen output module as a set point value. A light output is measured and a measured value is subtracted from the set point value provided by the lumen output module to provide a difference or error signal. The error signal is supplied to a lumen output controller, which adjusts a pulse width modulation (PWM) of power supplied to LEDs of the corresponding distinct color. Thus, the first, feed forward junction temperature dependent part and the second, lumen feedback part are connected in series. With such a controller the output of emitted light is controlled to be identical to a set point value supplied by the lumen output module of the feed forward part.
A controller, which provides feed forward junction temperature compensation only, can be used to compensate for differences of light output and wavelengths shifts due to changes of junction temperature(s).
A controller, which comprises a lumen feedback to control a lumen or light output only to be identical to some set point value, could be used to compensate for changes of light output due to temperature effects and aging of the LEDs.
The prior art controller comprises an algorithm for the feed forward part and the feedback part, which includes many calculation steps. The temperature of the LED arrangement may vary rather fast, and, as a consequence, light output power and wavelengths shift also. Therefore the calculation of such algorithm must be carried out with a high pace, which, in practice, is identical to a pulse width modulation period at which a supply to the LEDs is modulated. To avoid visible flickering in the light output of the module, the pulse width modulation period is usually shorter than 20 milliseconds. As a consequence, a processor for carrying out said calculation must be powerful and therefore will be expensive. A complicating factor is that when using a single light sensitive element to measure the light output of each color, it is required to time shift the on-time for each color. It also requires the use of a minimum on time for each color during each PWM period, so that the combined light output of all colors always contains a fraction of each color. To minimize such fractions and thereby maximizing the control range of light output for each color, the light output for each color must be sensed and evaluated even faster, which requires an even more powerful and expensive processor.
The inventors found that compensating the light output for changes due to aging need not to be carried out with such high pace. In addition, the inventors conceived that an output of a feed forward junction temperature compensating part should not be used as such to provide a set point for the wanted light output.